This disclosure relates to a fuel supply system, and more particularly to a filter assembly for fuel fed to actuators and a fuel control system of an aircraft engine, and associated method. More particularly, this disclosure relates to a fuel supply system that employs a wash flow filter assembly, and a means for providing sufficient velocity through the filter to effectively remove material collected on the wash flow filter.
In an aircraft engine, there are times in which the main fuel pump has a substantial amount of flow, while in other instances there is significantly less flow through the pump. Pressurized fuel proceeds from the pump through a simple valve, sometimes referred to in the industry as a system stability valve, that breaks up any feedback between the remainder of the system and pump operation. For example, when a centrifugal pump has a positive sloping head curve there is some instability at lower flow rates while at higher flow rates the system is more stable. Jet engines require operation in a low flow situation so a valve is incorporated into the system downstream of the fuel pump to break up or limit feedback and work better in low flow conditions.
The fuel flow is often split between a fuel control system where the fuel is burned or combusted and for moving actuators. A main filter is intended to filter particle sizes down to 20 microns, although there is a need for a backup filter since assurance of filtered flow to a particle size on the order of 40 microns is important to protect the actuation system that includes, for example, servo valves, etc. Usually, system specifications want a filter that does not have to be serviced. The system is usually pumping more fluid than is needed and thus there is also sufficient flow provided to the wash flow filter to remove the material that collects on the filter surface. However, if a variable pumping system is used, then the system loses the benefit of the high wash flow and there will be times when low wash flow is provided. It is also desirable that the auxiliary filter be small and lightweight, and able to handle a wide range of wash flow
It is known that when an actuator holds a position, then generally steady state flow is provided. On the other hand, when the actuators are moved quickly, a lot of flow (e.g., typically greater than four times the steady state flow level) is provided to the actuators. Thus, filtered flow through the wash flow filter can be widely varied and varied relatively quickly. Typically, the amount of fuel that passes through the wash flow filter is not an issue. At engine idle power the amount of flow may be quite low that is delivered to the fuel control system. This flow is low in comparison to the flow to the actuators (actuator flow may be on the order of four times greater). Normally, when a wash flow filter is used, the through flow to the fuel control system would be typically greater than the filtered flow. For example, the through or unfiltered flow would be on the order of five to ten times greater than that of the filtered flow that passes through the filter. In such situations, a low amount of through flow (here, fed to the fuel control system) may result in insufficient wash flow through the filter so that plugging of the filter can result.
During times of system operation encountering such low through flow, a device is needed that can provide increased velocity through the filter, particularly where the flow to the fuel control system is relatively low. A need exists to provide a simple, effective, and cost efficient manner of maintaining sufficient velocity wash flow through the filter at all times to assure good washing of the filter screen with a minimum amount of complexity and service. It is also desirable if such a system could be easily incorporated into known fuel delivery mechanisms.